The present invention relates to a drum cooker for cooking granular products, especially corn grits for the production of cornflakes, having a vessel mounted to rotate about an axis with nozzles that open into the vessel and that are connected with lines through which a gaseous heat-carrying medium can be introduced into the vessel, there being provided at least two circumferentially distributed nozzles so that at least one nozzle opens into the area covered by the product while at least one nozzle opens into the area not covered by the product.
A drum cooker of this type is known, for example, under the designation DSCK from Gebr. Buhler AG, Uzwil, Switzerland.
A comparable drum cooker is also known from APV Baker, Peterborough, England, under the designation "Rotary cereal cooker".
The axis of the rotary vessel extends horizontally, the vessel as such has a substantially barrel-shaped contour. Inside the vessel, the product to be treated, for example corn grits, is mixed with water, the vessel being filled with that product-water mixture up to more than half its volume.
The heat supply is realized by nozzles arranged in the wall of the vessel, that are supplied with a hot gaseous medium, namely hot water steam, through lines extending along the outside of the vessel. The arrangement of the nozzles is such that the nozzle groups are located substantially diametrically opposite one to the other. All lines and/or nozzle groups are supplied form a common supply source via communicating lines. This means that the steam is supplied to all lines and all nozzles simultaneously.
In practical use it has, however, been found that the steam enters the interior of the vessel only through those nozzles that open into the area not covered by the product-water mixture at that time. These are the nozzles that occupy a position above the product-water mixture during the rotary movement.
Those nozzles that open into the area just covered by the product-water mixture hardly admit any steam since the counterpressure of the product-water mixture present in the vessel is so high as to prevent the steam from entering through those nozzles in any quantity worth mentioning.
This is due to the fact that the gas, being supplied simultaneously via all lines, tends to follow the line of least resistance, which is through the nozzles that are not immersed in the product-water mixture.
Even increasing the pressure does not cause the gas to enter through the nozzles in the area covered by the product.
This now leads to the effect that very poor heat transmission is achieved from the admitted heat-carrying medium to the product-water mixture revolving in the vessel, i.e. the mixture of water and the corn grits to be treated. The heat exchange occurs substantially at the surface of the product, which in revolving condition has a kidney-shaped cross-section, and the upper steam-filled space.
However, since it is necessary to continuously supply fresh heat-carrying medium for the cooking process, additional arrangements must be provided for carrying off the gaseous medium from the vessel, which is effected through openings arranged above the product-water mixture. This then creates the risk that fresh medium is carried off without a considerable heat exchange having taken place with the revolving product-water mixture.
This is connected with the considerable disadvantage that very long cooking times are necessary, with the risk that the starch shows gelatinizing phenomena in the outer areas of the corn grains, that have been cooked for a long time, while inside the grain the cooking process has not yet led to the desired result, so that a so-called "white spot" of uncooked starch is still left.
During further processing, this then leads to the result that after the grains have been rolled to flakes, these contain nongelatinized spots. The roasting process that follows then leads to cornflakes which exhibit pale discoloring in the area not thoroughly cooked (white spot), and which instead of containing the desired blisters, exhibit in this area a baked structure, i.e. a closed structure similar to puff pastry.
With respect to energy balance, this cooking process is extremely unfavorable.
It is, therefore, the object of the present invention to improve a drum cooker of the before-mentioned kind so that an efficient heat exchange and/or cooking process takes place that leads to reduced cooking times and by which, in addition, a good and uniform cooking result is achieved.